Rotary indexing tables are designed to accommodate a wide range of machining and inspection applications. Rotary indexing tables are of two general types.
The first type utilizes a bearing (either a roller or ball bearing type) to oppose the machining forces. However, roller and ball bearings in a static condition are considered soft (permitting a high rate of deflection) and provide increasing stiffness only as the load increases. Basically, this stiffening effect is caused by the fact that minimal, initial contact area increases in size as the load increases. The usual method used to minimize this effect is to preload the bearing. However, preloading bearings increases friction, causes fatigue and decreases bearing life. Therefore, in rotary indexing tables utilizing bearings, a design compromise must be made which ultimately compromises rigidity of the rotary indexing table.
The second type utilizes a serrated tooth, or Hirth type coupling, which is machined as an integral part of the base and top plate of the rotary indexing table. In its simplest form, a device of this type consists of two 25 serrated tooth plates having the same number of teeth and shape so that one plate can be tightly engaged with the other plate. When the teeth are intermeshed, the plates cannot rotate with respect to each other. However, the angular position of the two plates can be changed by either lifting the top plate with respect to the other, or lowering the bottom plate relative to the top plate, rotating the top plate and, thereafter, re-engaging the teeth to position the top plate in a predetermined angular relationship relative to the bottom plate.
Rotary index tables utilizing the serrated tooth coupling have a number of advantages. For example, there are no joints other than the coupling between the base and the table top. Therefore, all machining forces, such as bending moments and torques, are transmitted from the top plate to the base through the coupling. Thus, the degree of rigidity of the face plate position is directly proportional to the force applied to hold the coupling together and the diameter of the coupling. As the locking force and the diameter of the coupling is increased, the degree of rigidity of the device increases.
Rotary index tables utilizing the aforementioned serrated tooth coupling are also advantageous for rotating work pieces that are being machined or inspected into discrete positions and retaining them firmly in the position selected. For example, if two plates each have 360 teeth, the top plate can be rotated to allow the positioning of a work piece thereon in any of 360 possible positions. This precision arises due to the averaging effect of the serrations, in which the machining inaccuracies of the serrated teeth are averaged out over the entire ring series, to thus provide a precision which is much greater than that with which the individual gear teeth can be manufactured. Another characteristic of the Hirth type coupling is that accuracy tends to improve with wear since the teeth become more and more uniform after repeated indexing cycles.
A rotary index table employing a single ring of serrated teeth is physically limited in the number of teeth that can be formed thereon. This severely limits the subdividing angular resolution achievable. One solution to this problem is the provision of an additional rotary member supported on independent bearings, with the additional rotary member being coupled to the two mating plates. However, the additional coupling and the required bearings are expensive to produce for extreme accuracy. Futhermore, this approach generally results in a fixture which is not as rigid or accurate as desired in such applications.
One attempt to solve this problem is shown in U.S. Pat. No. 3,846,912 issued to Richard J. Newbould on Nov. 12, 1974. Newbould discloses an indexing mechanism utilizing a plurality of plates connected by locking teeth. The discs, or linear plates, are provided with a series of intermeshing teeth between the intermediate adjacent surfaces of the plates. The plates are designed for relative rotation about an axis normal to the planes of the plates. The plates are adapted to be separated in an axial direction to permit relative, independent and desired positioning of the plates relative to a base reference indicia. The initial major divisions of an angle are provided by the position of one of the plates. Smaller divisions of the selected angle are provided by another of the plates. Still smaller divisions of the selected angle are provided by a third plate. It is, however, essential to the effective operation of this mechanism that at least one of the pitches between a pair of meshing plates is not evenly divisible into a circle. Therefore, this necessitates a clearance, or a clearance area, in one of the plates to accommodate nonmeshing or improperly meshing teeth to allow the necessary movement used in subdividing on the other pitches. These clearance areas, however, reduce the locking force available to resist the rotational forces imparted on the top plate by the machining operation. Furthermore, these clearance areas necessitate that the intermeshing teeth on the one plate be segmented. This reduces the number of teeth available on the ring for averaging the tooth error and thus reduces the accuracy of the device. Also, Newbould requires the addition of more plates to achieve greater subdividing with a subsequent loss of system accuracy. Therefore, Newbould cannot generate the locking force inherent in a ring of equally spaced intermeshing teeth and is not able to take advantage of the averaging effect of a ring of equally spaced serrated teeth even with extended usage. Thus, the Newbould device is not suitable for very accurate applications.
Another device utilizing intermeshing teeth is disclosed in U.S. Pat. No. 3,961,780 issued to Joseph J. Saj on June 8, 1976. Saj discloses a rotary indexing table wherein a work support table is rotatably journalled on a base. The plate and base each have a downwardly facing ring of teeth thereon. An axially movable coupling ring has two rings of teeth thereon which register axially. The rings of teeth on the plate, and the rings of teeth on the base, are adapted to mesh with each other to fixedly retain the base at a predetermined angular position relative to the base. A vernier angular adjustment of the top plate relative to the base is provided through an arc motion utilizing a fraction of the spacing between successive teeth on either of the two rings. This angular adjustment is provided by rotating the piston by means of a handle extending through a radially extending slot in the bottom retainer ring. This severely limits the rotation of the ring and the number of angular positions available for the vernier operations.
The accuracy of the Saj device also depends on the accuracy to which the teeth in the two sets of teeth are formed and also upon the ability of the flat inclined faces of the teeth in the two sets of rings to mesh perfectly. Unless the teeth of both rings mesh perfectly, precise positioning will not be achieved and the required extreme accuracy of the indexing operation will not result. To assure perfect meshing of the teeth, Saj weakens his coupling member or piston by a groove. Saj also utilizes a ring to serve as a reaction member which, when pressure is applied to the cylinder, applies a downward force on the ring and thus tends to draw the teeth on the ring downwardly into meshing engagement with the teeth, thereby compensating for any misalignment of the teeth. While this misalignment of the teeth does not adversely affect the rigidity for light machining operations, this arrangement does not provide adequate rigidity in applications with higher machining tool forces.
The basic design of Saj's device also provides for an inherent conflict between the ball bearings which support the top plate for rotation in a horizontal plane and the intermeshing teeth of the coupling. This conflict reduces the effectiveness of the intermeshing with the coupling so that accuracy and the rigidity of the device is further compromised.
Therefore, none of the above-identified prior art rotary index devices provide both the rigidity required to oppose the higher machining forces necessary in many new applications and the desired greater subdividing resolution required.